Recently, an MRAM (Magnetic Random Access Memory) device has attracted attention as a new generation nonvolatile memory device. The MRAM device is a nonvolatile memory device storing data in a nonvolatile manner with a plurality of memory cells including magnetic thin films formed on a semiconductor integrated circuit, and allowing random access to each of the memory cells.
Generally, such a memory cell includes a magnetoresistive element with a spin valve structure in which a pinned layer made of a ferromagnetic layer having a pinned magnetization direction and a recording layer made of a ferromagnetic layer having a magnetization direction changed according to an external magnetic field are arranged with a nonmagnetic layer interposed therebetween. The magnetoresistive element with the spin valve structure stores data to correspond to a change in an electric resistance value generated in response to a change in the magnetization direction of the recording layer. Changes in an electric resistance value are classified into the tunnel magnetoresistive effect, the giant magnetoresistive effect, and the like, according to the principles thereof. It has been known that using a magnetoresistive element utilizing the tunnel magnetoresistive effect drastically improves the performance of the MRAM device.
Most magnetoresistive elements are formed to have magnetic anisotropy such that the magnetization direction of the recording layer is parallel or antiparallel to the magnetization direction of the pinned layer, and binary values “0” and “1” are stored to correspond to the magnetization direction of the recording layer. A direction parallel or antiparallel to the magnetization direction of the pinned layer as described above is referred to as an easy axis of magnetization of the recording layer, and a direction orthogonal to the easy axis of magnetization is referred to as a hard axis of magnetization. Specifically, the magnetization direction of the recording layer is switched alternately on the easy axis of magnetization according to an external magnetic field. Such a characteristic by which the magnetization direction of the recording layer is oriented to either direction on the easy axis of magnetization is referred to as uniaxial magnetic anisotropy. Uniaxial magnetic anisotropy is implemented by shape anisotropy caused by lengthening an in-plane shape of the magnetoresistive element along the easy axis of magnetization.
When an MRAM device is formed of memory cells including such magnetoresistive elements, two types of write lines are arranged in rows and columns, and a memory cell is arranged at a position adjacent to each intersection of the two types of write lines. Each memory cell is arranged to be subjected to an external magnetic field in the direction of the easy axis of magnetization and an external magnetic field in the direction of the hard axis of magnetization generated by currents flowing through the two types of write lines, respectively. The magnetoresistive element constituting the memory cell switches the magnetization direction of the recording layer alternately according to a synthetic magnetic field generated by the external magnetic field in the direction of the easy axis of magnetization and the external magnetic field in the direction of the hard axis of magnetization. The orientation and the magnitude of the synthetic magnetic field switching the magnetization direction of the recording layer as described above are referred to as asteroid characteristics, and defined by the magnitudes of the external magnetic field in the direction of the easy axis of magnetization and the external magnetic field in the direction of the hard axis of magnetization. Specifically, in the MRAM device, any one of a plurality of memory cells arranged in rows and columns is specified by appropriately selecting the two types of write lines adjacent to a specific memory cell and passing currents therethrough, thus implementing random access.
It has been known that a magnetic field required to switch the magnetization direction of the recording layer (hereinafter also referred to as a switching magnetic field) is determined by the shape of the recording layer, and is substantially inversely proportional to the width of the recording layer in an in-plane direction and proportional to the thickness of the recording layer, as disclosed in “Submicron spin valve magnetoresistive random access memory cell” by E. Y. Chen et al., Journal of Applied Physics, vol. 81, No. 8, pp. 3992-3994, 15 Apr. 1997 (Non-Patent Document 1). Therefore, when an attempt is made to further miniaturize a memory cell to implement a further highly integrated MRAM device, the width in the in-plane direction is to be reduced and the switching magnetic field is to be increased, causing an increase in power consumption during data write. Accordingly, the recording layer is formed into a thin film to suppress the increase in power consumption during data write. However, forming the recording layer into a thin film has limitations, which may be a factor inhibiting the implementation of a further highly integrated MRAM device.
Consequently, a magnetoresistive element having the same length in the direction of the easy axis of magnetization and in the direction of the hard axis of magnetization and thus having no shape anisotropy to suppress an increase in the switching magnetic field has been proposed for example in “Size-independent spin switching field using synthetic antiferromagnets” by K. Inomata et al., Applied Physics Letters, vol. 82, No. 16, pp. 2667-2669, 21 Apr. 2003 (Non-Patent Document 2), and “Magnetization reversal and domain structure of antiferromagnetically coupled submicron elements” by N. Tezuka et al., Journal of Applied Physics, vol. 93, No. 10, pp. 7441-7443, 15 May 2003 (Non-Patent Document 3). To implement uniaxial magnetic anisotropy in such a magnetoresistive element without shape anisotropy, it has been proposed to employ an antiparallel coupling structure (i.e., Synthesis Anti-Ferromagnetic structure; hereinafter also referred to as an SAF structure) including two ferromagnetic layers exchange-coupled in an antiparallel manner with a nonmagnetic layer interposed therebetween.    Non-Patent Document 1: “Submicron spin valve magnetoresistive random access memory cell” by E. Y. Chen et al., Journal of Applied Physics, vol. 81, No, 8, pp. 3992-3994, 15 Apr. 1997    Non-Patent Document 2: “Size-independent spin switching field using synthetic antiferromagnets” by K. Inomata et al., Applied Physics Letters, vol. 82, No. 16, pp. 2667-2669, 21 Apr. 2003    Non-Patent Document 3: “Magnetization reversal and domain structure of antiferromagnetically coupled submicron elements” by N. Tezuka et al., Journal of Applied Physics, vol. 93, No. 10, pp. 7441-7443, 15 May 2003.